Rock groups

When I walk on the beach, I notice that rocks of the same size seem to be clustered together. All the big ones are together, and all the small ones are with other small ones.

-- Wanderer, Solana Beach

While you're padding across the sand, the beach itself is moving too. In Southern California, sand and rocks are continually heading south with the prevailing longshore currents. During times of heavy surf in the winter, the veneer of sand is stripped off the cobblestone base that makes up many of our beaches. New sand is redeposited by the gentler waves of spring and summer. But on a smaller scale, individual rocks are being shoved around too. Rocks of roughly the same size will congregate because it takes the same amount of wave energy to get each one of them moving (the erosion velocity) and keep them moving. When the energy drops below a certain point (the fall velocity), rocks of the same size (actually, the same weight) will stop moving at roughly the same time. Enough of this pushing and shoving, and you'll eventually see the tide line covered with clumps of cobbles that look about the same. Beach contour and some other factors also affect where sand and rocks end up, so this is the shorthand version for you folks who are saying, "Aw, who cares, Matt? Let's get on to something juicy."